Photographic silver halide light-sensitive materials

ABSTRACT

WHEREIN Z, R, R1, L1 and L2 are as defined in the specification.   A silver halide photographic light-sensitive material wherein one emulsion layer contains silver halide grains with a mean grain size no larger than 0.9 microns, at least one group VIII metal compound, and at least one sensitizing dye of the formula:

United States Patent [1 1 Shiba et al.

PHOTOGRAPHIC SILVER HALIDE LIGHT-SENSITIV E MATERIALS Inventors: Keisuke Shiba; Masanao Hinata;

Koutarou Yamasue; Akira Sato; Tadashi Ikeda, all of Kanagawa, Japan Fuji Photo Film Co., Ltd., Kanagawa, Japan Filed: Dec. 30, 1971 Appl. No.: 214,216

Assignee:

Foreign Application Priority Data Dec. 30, 1970 Japan 45-l23036 US. Cl 96/125, 96/108, 96/140 Int. Cl... G03c l/08 Field of Search 96/125, 108, 140

References Cited V UNITED STATES PATENTS 2,078,233 4/1937 Brooker 96/140 8/1948 Smith et a1 96/108 8/1951 Trivelli et al. 96/110 51 Feb. 5, 1974 11/1969 Kampfer et al. 96/140 FOREIGN PATENTS OR APPLICATIONS 1,074,433 7/1967 Great Britain 96/ 140 Primary Exan/iner-J. Travis Brown Attorney, Agent, or Firm-Richard C. Sughrue et a1.

wherein R, R L and L are a s defi ned in the specification.

15 Claims, 1 Drawing Figure Pmamzorca M 3.790.390

PERCENT TRANSMISSION WAVELENGTH (n m) PHOTOGRAPHIC- SILVER HALIDE LIGHT-SENSITIVE MATERIALS BACKGROUND OF THE INVENTION tion age, various systems for rapidly communicating information have been developed. For example, there are press facsimile systems for rapidly sending newspaper manuscripts to remote places, high speed. photographic typesetting'systems for. rapid typesetting, cathode raytube display systems for indicating the information output of a computer in letters or in patterns, and the like.

In these instruments for rapidly communicating information, short-time exposure, less than l/l00,000 second, sometimes as short as about 1/ 1,000,000 second, is often employed. Recently, the light-sensitive materials for use with such instruments have been more and more in demand.

As light sources used in these instruments, there have been used combinations of light sources having high illuminance, such as a xenon arc lamp, and high speed shutters; xenon flash light-sources; and cathode raytube light sources. Amount these, cathode ray-tubes, especially those having fluorescent substance with short-time afterglow, are generally used for flying spot. For example, various fluorescent substances referred to as P-IS, P-l 1-, P-16, P-24 etc., are used, all of which are well known to one of ordinary skill in the art, Among these, P-l5, P-lli, P-l6, and P-24 are known to have their maximum spectral energy distribution of emission at 505 nm, 460 nm, 385 nm and 520 nm, respectively. It is necessary in a light-sensitive material for recording images formed ona cathode raytube wherein these various kinds of fluorescent substances are used to have a high sensitivity in at least the blue light-sensitive region, especially at the wavelengths of from 380 to 520 nm.

The afterglow time of the emission of the aforesaid cathode ray-tube is usually as short as 1/l0,000,000 to 1/l00,000 second. Xenon flash lamps having an emission time similarly short are frequently used. In the present specification, the above-mentioned short-time exposure by a light source of this kind is generically referred to as flashlight exposure.

The room wherein such light-sensitive materials are used is generally comparatively bright so that the operation can be conducted with ease. These materials are often used under comparatively bright yellowish-green safety light such as a Fuji Safe Light No. 2A (the percent transmission curve thereof is given in the drawing); and hence light-sensitive materials having a low sensitivity to safety lights of this kind are desirable.

SUMMARY OF THE INVENTION- Oneobject of the present invention is to provide a silver halide light-sensitive material having a high sensitivity to blue light in flashlight exposure.

Another object of the present invention is to provide a light-sensitive material having high sensitivity to blue light which is capable of being dealt with under bright yellowish-green safety light. l

The blue-light-sensitive region (mainly the wavelength region of from 380 to 520 nm) corresponds to the light-sensitive region intrinsic to silver halide. In order to obtain a silver halide emulsion having high intrinsic sensitivity in flashlight exposure, processes for increasing the sensitivity by making the grain size of the silver halide large, processes for increasing the sensitivity by chemical sensitization, etc., were investigated. However, in the process comprising making the grain size large, the granular properties and the sharpness of the image formed were greatly deteriorated. In the process of chemical sensitization, the stability thereof to v the safety light was deteriorated. In addition, when the propyl, 2-(2-carboxyethoxy) ethyl), sulfo group containing alkyl (e.g., 2-sulfoethyl, 3-sulfopropyl, .4-

sensitivity of the silver halide was increased by chemical sensitization in flashlight exposure, the storage stability thereof was deteriorated and, for example, undesirable fogs or stains .were observed. The objects of the present invention were attained by using a silver halide emulsion containing light-sensitive silver halide grains whose mean grain size by diameter (and by number) is equal to or smaller than 0.9 micron; at least one compound containing .a group VIII metal in the periodic table in an amount of 10 to 10' mol per 1 gram mol of the silver halide; and at least one sensitizing dye represented by the general formula:

wherein R and R each represents a hydrogen atom, an alkyl group, a substituted alkyl group or an aryl group, Y represents an oxygen atom or N R,, (R being a hydrogen atom, an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group or an allyl group), L and L each represents a methinegroup and Z represents the non-metallic atoms necessary to complete a series of oxazoline nuclei; in a binder such as gelatin.

BRIEF DESCRIPTION OF THE DRAWING The drawing is the percent transmission curve of a bright yellowish-green safety light.

DETAILED DESCRIPTION OF THE INVENTION sulfobutyl, Z-hydroxyl -sulfopropyl, 2'( 3-sulfopropoxy)ethyl, Z-aetoxy-l-sulfopropyl, 3-methoxy-2-(3- sulfopropoxy)propyl, 2-[2-(3-sulfopropoxy) ethoxy]ethyl, 2-hydroxy-3--(3'-sulfopropoxy) propyl), aralkyl (e.g., benzyl, phenethyl, p-sulfobenzyl, pcarboxybenzyl), allyl group, etc., or an aryl group such as phenyl, preferred carbon atom range of said alkyl moiety being up to 8; Y represents an oxygen atom or NR (wherein R represents a hydrogen atom, the same alkyl group as R and R,, an aryl group (i.e. unsubstituted aryl group and substituted aryl group) such as phenyl, tolyl, naphthyl, etc., alkoxy phenyl (e.g., methoxyphenyl), halogenophenyl (e.g., chlorophenyl), etc.), L and L each represents a methine group preferably having up to 8 carbon atoms such as =CH, =C(CH =C(C H =C(C l-l etc., and Z represents the non-metallic atoms necessary to complete a series of oxazoline nuclei such as the oxazoline nucleus, 4- and/or 5- alkyl substituted oxazoline nucleus (e.g., 4,4-dimethyloxazoline nucleus, 5- methyloxazoline nucleus, etc.), 4- and/or S-phenyl substituted oxazoline nucleus (e.g., 4,5-diphenyloxazoline nucleus).

The sensitizing dyes used in the present invention are known and are described, for example, in British Pat. No. 1,074,433, and can be readily synthesized by reference to said specification.

Examples of dyes represented by the general formula are set out below, but the dyes used in the present invention are not to be thereby limited.

Dyel 0 L C=CHCH=CO H N/ O \N/ S 3H: (I) H D 2 O Y H,/

C=CHCH=C-O (EH8 i711:

Dye3 O\ C=CHOH=E-g H I( 1/ O \N/ S Ha HzQgHs Dye4 O 2 C=OHCH=CO o= (i=5 N 1H:

Dye5 O H 0213s I G=CHOH=CI(:-: HO Hi0 N O S Ha (ilzHa yefi OH l5 Dye9 Dye 8 Dye 7 O N CH: i

Dye 11 O H2 HaC H3O N H, CH: O H

Dye 12 O mide, silver bromoiodide, etc. The grain size of the sil- .ver halide is, more effectively, smaller than 0.8 micron, most especially not larger than 0.6 micron, on the average.

The production of the light-sensitive silver halide grain, that is, the production of the silver halide emulsion, comprising forming the crystals of silver halide grains in a dispersed state in the binder, and developing the crystals into a suitable grain size, is conducted according to conventional methods.

The group Vlll metals of the periodic table include iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum and, as the compounds containing these metals which are rnost preferably used in the present invention, there are iron, iridium and rhodium compounds. As examples of the group VIII metal compounds according to this invention; there are ferrous sulfate FeSO -5H ferric chloride FeC1 potassium hexacyanoferrate (II) K Fe(CN) '3I-I O; po- 5 ous chloride NiCl '6H O; nickel (II) nitrate Ni(- NO -6H O; ruthenium (Ill) chloride Rucl potassium hexachloror'uthenate (IV) K RuCl rhodium (Ill) chloride RhCI '4I-I O; ammonium hexachlororhodate (III) (NI-l RhCl palladium (II) chloride PdCl palladium (ll) nitrate Pd(NO palladium (II) bromide PdBr,; potassium hexachloropalladate (IV) K PdCl potassium tetrathiocyanatopalladate (II) K Pd(CNS) osmium (ll) chloride OsCl,; iridium (Ill) chloride lrCl iridium (IV) chloride IrCl iridium (Ill) bromide IIBI'3'4HQO; iridium (IV) bromide lrBr potassium hexachloroiridate (III) K .,rrc1 potassium hexachloroiridate (IV) K lrCl ammonium hexachloroplatinate (IV) (NI-I PtCl potassium hexachloroplatinate (IV) K PtCI ;v ammonium j hexabromoplatinate (IV) (NH PtBr and the like. These group VIII metal compounds are used in an amount of 10 mol to 10' mol per 1 mol of silver halide, independently or in combinations of more than one of these compounds. A gold compound can be preferably added to the emulsion in combination with any of those compounds of the group VIII metal.'The addition of these compounds is properly' conducted at the formation of the silver halide grains, at the start of the ripening of the emulsion or in the course thereof in the production of the silver halide emulsion. Otherwise, they may be. added, before coating the ripened emulsion on a support, to the emulsion together with -a known stabilizer such as 2- mercaptobenzimidazole, I I l-phenyl-5 mercaptotetrazole, 5-methyl7-hydroxy-l ,3,4- triazoindolizene, etc., antifogging .ag'ents, toning agents, coating assistants such as sodium alkylbenzenesulfonate, saponin, etc., plasticizers, hardeners such as 1 formaldehyde, mucochloric acid, delustering agents, development accelerators, fluorescent brightening agents, color couplers or developing agents for activator development, etc. These materials are added,,when desired, for their'art-recognized function.

The sensitizing dyes used in he present invention are used in an amount of 10".to 10 mol per mol of silver halide contained in the silver halide-emulsion, either,

separately or in combinations of more than one of them. The addition of the sensitizing dyes to the emulsion is carried out inthe. same manner as the aforesaid metal compounds. They may-be added to the emulsion simultaneously with the said metal compounds, or may be added independently regardless of the order.

' After the flashlight exposure, the light-sensitive material prepared in this way can be developed and fixed in the conventionalmanner, or may be treated in the manner generally known as the diffusion transfer process, or may be treated according to the developing yantageously not larger than 0.9 micron.

It has been known that, regardless of the existence of the above-mentioned metal compounds, the sensitizing dyes represented by the general formula are capable of spectrally sensitizing light-sensitive materials. As the result of various experiments in flashlight exposure, the sensitizing effect of the emulsion not containing abovementioned metal compounds is small while in the usual exposure, the emulsion not containing said metal compounds shows at least about the same sensitizing effect as that of the emulsion containing said metal compounds. However, surprisingly, in the emulsion containing the group VIII metal compounds of the present invention, a remarkable sensitizing effect is produced as is shown in the following examples. On the other hand, the light-sensitive material whose sensitivity in flashlight exposure is increased by chemical sensitization or by the process whereby the grain size is made larger, has an increased sensitivity to safety light, and under a safety light is deteriorated, as is shown in the -following examples. Especially, the mean grain size of the silver halide is quite important for use under a safe light, and the mean grain size of the silver halide is ad- A bright safety light is desirable and it is the prerequisite for the light-sensitive material used in the present invention that it can be safely dealt with under a yellowish-green safety light, and hence it can of course be dealt with under red light, such as a Fuji Safe Light No. 2, or under'the light passed through a safety light filter which transmits light of longer wavelengths.

It has long been known that, when added to a photographic emulsion, theabove-mentioned metal com pounds exhibit various effects. As is described in US. Pat. 2,448,060, when hexahalo'genoruthenate .(III), hexahalogenorhodate (IV),v hexahalogenopalladate (IV), hexahalogenoosmate (IV), hexahalogenoiridate (IV), hexahalogenoplatinate (IV), etc., are added to a silver halide emulsion upon ripening, the intrinsic sensitivity of the light-sensitive material wherein this emulsion is used increases and, as is described in US. Pat. No. 2,566,245, when said compounds are added to a silver. halide emulsion in large amounts, the storage stability of the light-sensitive material obtained under the circumstances of high temperature and high humidity is improved. Furthermore, it is described that potassium hexacyanocobaltate (Ill) (see US. Pat. .No.

various photographic effects may be obtained at the same time. In addition, as described in British Pat. No. 570,393, a process comprising adding a gold ,compound upon ripening to raise the intrinsic sensitivity of the light-sensitive material can also be '60 a p ied.

Thepresent invention'will now be further illustrated .in more detail by the following examples.

EXAMPLE I A silver bromoiodide emulsion"ub'fittiiiiifig"27561 percent of Agl) of about 0.6 micron mean grain size,

not containing the compounds of a group VIII metal (hereinafterreferred to merely as a metal compound" veloper for 3 minutes at 20 C.

u in the examples) [control emulsion (I) was prepared in the presence of l X 10' mol of ammonia per 1 mol of silver halide, and an emulsion having about the same sensitivity as that of the control emulsion (l) was prepared with the same prescription with adding 3 X 10 5 mol of ammonium hexachlororhodate (Ill) per 1 mol of silver halide contained in the said emulsion upon. forming the silver halide grains and further adding 2 X mol of potassium chloroplatinate per 1 mol of silver halide upon post-ripening. Each of these two emulsions was divided into two portions, then 6 X 10 mol of the Dye l per 1 mol of silver halide contained in the emulsion was added to one portion, leaving the other portion as such. in addition, 2-mercaptobenzimidazole was added thereto as a stabilizer, mucochloric acid as a hardener, and saponin as a coating assistant, and the emulsion was then coated on a polyethylene terephthalate (PET) film.

On the other hand, 8 X 10 mol of ammonia per 1 mol of silver halide was added upon precipitating the silver halide to prepare an emulsion of 0.95 micron mean grain size (containing the above-mentioned metal compound, but free from the dye), and the emulsion coated on a PET film base (which corresponds to control emulsion (l')).

The above-described samples were imagewise exposed for 1/100 and l/l,000,000 second using a Mark Ill sensitometer made by EG G Co. in the U.S.A. The exposed samples were developed with the following de- Water to make up 1 liter mined as the reciprocal of the quantity of light which After fixing, washing and drying, the transmission density was measured. The sensitivity value was deter- 40 gave a density of 2.5. The sensitivity of the control emulsion (I) was calculated as 100, and the relative sensitivities of the other emulsions based on that of the control emulsion (I) are given in Table l.

In addition, the fog density value obtained by leaving the me? d J F iiNQ Aa sk 3ht (2936 12! 8 distance, &1. m e -.1 mi nZQniaussssQhss developing them with the above-described developer, was also measured.

Vlll metal+ Dye l Control emulsion 150 v emulsion to flashlight is remarkably increased by the combined use of rhodium salt, platinum salt and the Dye i'farfi the safetytothe saletyTi g'htis not so poor. It is also seen that, in the emulsion whose grain 'size was made big to obtain about the same sensitivity, the safety thereof under the safety light deteriorated.

' EiAlIiEi- A silver chlorobromide emulsion (containing 50 mol percent of AgCl and 50 mol percent of AgBr) of about 0.6 micron mean grain size, not containing the metal compounds of the present invention [control emulsion (II)], was prepared in the presence of 10 mol of ammonia per 1 mol of silver halide, and an emulsion having substantially the same sensitivity as said control emulsion (ll) was prepared with the same prescription but with further adding 6 X 10" liiol of potassium hexachloroiridate (IV) and 3 X 10" mol of ammonium hexachlororhodate (lll) per 1 mol of silver halide upon forming silver halide precipitates, andfurther adding 3 X 10 mol of potassium chloroaurate' thereto upon post-ripening. I

In the same way as in Example 1, 2 X 10 mol of the heretofore set out Dye 2 per 1 mol of silver halide was added thereto, then a stabilizer,'a hardener, a coating assistant, etc., were added and the emulsion coated on a PET film. a

On the other hand, a silver halide emulsion with a 1 micron mean grain size containing said metal compounds but free from the dye, prepared in the presence of 2.0 X 10f mol of ammonia per 1 mol of silver halide (control emulsion (ll')), was similarly coated onto a PET film. The sensitivity and the fog under a No. 2A Safe Light were measured in the same manner as in 'Example l.

TABLE 1 TABLE '2' i Fog when Sensitivity Sensitiv- Fog when exposed exposed for ity exposed gay i h e i t 1/ 2A Sensitivity Sensitivity zgw/g r w Safe exposed for exposed forf 1/l,000,000 light for: 1/100 sec. 1/ l ,000,000

sec. 1 min. 20-min. "Q l min. 20 min.

Control emulsion loo 100 0.0 0.03 g' 0 (l) Control emulsion loo 105 0 0.01 Control emulsion 98 100 0.0 0.02 921+ t e group (l) the group metal vm metal v I v fi?)nf+i:OIl);;nl2llSl0n I50 I15 0 0.03 Control emulsion 130 HO 0.0 0.04 Comm] emulsion 130 0 0 (I) D 1 (ll) the group Control emulsion I50 0.0 0.0l Dr 2 (I) the group Control emulsion I60 0.01 1.2

group VIII metal Dye 4 As is obvious from Table 2, a remarkable sensitizing effect to flashlight exposure was recognized by the combined use of the group -VIII metal salts and Dye 2 without deteriorating safety under a safety light, whereas emulsion (II') showed badly deteriorated safety under the safety light.

Example 3 An emulsion was prepared according to the procedure described in Example 2 using the Dye 3 instead of Dye 2, and tested in the manner described in Example 1. By the combined use of the group VIII metal salts and the Dye 3, a similar synergistic'effect was recognized.

Example 4 process, and to an emulsion prepared with the same prescription but with adding l X 10 mol of potassium hexacyanoferrate (III) per 1 mol of silver halide contained in the emulsion'upon forming the silver halide grains, and 7 X 10 mol of the Dye 4 per 1 molof silver halide in the same manner as in Example 1, there were v added, a stabilizer, a hardener and a coating assistant and the emulsions coated on PET films. The dye sensitization degree "of the samples thus obtained was measured in the same manner as in Example 1 to obtain the following results:

As is obvious from Table 3, the sensitivity to flashlight exposure is remarkably raised by the combined use of the group VIII metal and Dye 4. Substantially no r: ppear on the sameles. 2 the nosqttt ltv light-sensitive silver halide grains whose meangrain size by diameter is no greater than 0.9 micron; 10 to 10 mol of at least one of the compounds of the group VIII metals in the periodic table per 1 gram mol of said silver halide; and at least one of the sensitizing dyes represented by the following general formula:

wherein R and R, each represents a hydrogen atom, al-

kyl, or aryl group, Y represents an oxygen atom, N-R,,

(R being a hydrogen atom, alkyl, aryl, or allyl group),

L and L each. represents methine group, Z represents non-metallic atom group necessary to complete aseries of oxazoline nucleus 2. A silver halide photographic emulsion as claimed in Claim 1 wherein Y in the general formula is oxygen atom.

.3. A silver halide photographic emulsion as claimed in Claim 1 wherein the alkyl group vof R, R,, or R is a hydroxy alkyl group, an acetoxyalkyl group, an alkoxyalkyl group, a carboxy group-containing alkyl group, a sulfo group-containing alkyl group, an aralkyl group, or an allyl group, wherein the aryl group of R, is an alkoxy phenyl or a halogenophenyl group.

4. A silver halide photographic emulsion as claimed in claim I wherein the mean grain size is no greater than 0.6 micron.

5. A silver halide photographic emulsion as claimed in claim 1 wherein the group VIII metal is iron, iridium, or rhodium.

6. A silver halide photographic emulsion as claimed 3 in claim 1 wherein the emulsion further containsa gold compound. I

7. A silver halide photographic emulsion as claimed in Claim 7 wherein the gold compound is potassium chloroaurate.

8. A silver halide photographic emulsion as claimed 0 in Claim 1 wherein" theemulsion'further contains .2-

. mercaptobenzimid azole, l-phenyl-5-mercaptotetrazole or 5-methyl-7-hydroxyl ,3,4-triazoindolizine.

9. A silver halide photographic emulsion as claimed in claim 1 wherein the sensitizing dye is chosen from the group consisting of 10. A silver halide photographic emulsion as claimed in claim 3, wherein said hydroxyalkyl group is chosen I .in... .l.a. m wh rs ns id S 4 -sr9uPwntaini kyl.

1 l 12 group is selected from the group consisting of 2- I 15. A silver halide photographic emulsion as claimed sulfoethyl, 3-sulfopropyl, 4-sulfobutyl, 2-hydroxy-lin claim 3, wherein said a'ralkyl group is selected from sulfopropyl, 2-(3-sulfopropoxy) ethyl, 2-acetoxy-lhe group Consisting of benzyl, phenethyl, psulfopropyl, 3-methoxy- 2-(3-sulfopropoxy) propyl, lf b nzyh nd p-carboxybenzyl. 2-[2-(3-sulfopropoxy) ethoxy] ethyl, and 2-hydroxy-3- 5 (3'- sulfopropoxy) propyl. 

2. A silver halide photographic emulsion as claimed in claim 1 wherein Y in the general formula is oxygen atom.
 3. A silver halide photographic emulsion as claimed in claim 1 wherein the alkyl group of R, R1, or Ro is a hydroxy alkyl group, an acetoxyalkyl group, an alkoxyalkyl group, a carboxy group-containing alkyl group, a sulfo group-containing alkyl group, an aralkyl group, or an allyl group, wherein the aryl group of Ro is an alkoxy phenyl or a halogenophenyl group.
 4. A silver halide photographic emulsion as claimed in claim 1 wherein the mean grain size is no greater than 0.6 micron.
 5. A silver halide photographic emulsion as claimed in claim 1 wherein the group VIII metal is iron, iridium, or rhodium.
 6. A silver halide photographic emulsion as claimed in claim 1 wherein the emulsion further contains a gold compound.
 7. A silver halide photographic emulsion as claimed in claim 7 wherein the gold compound is potassium chloroaurate.
 8. A silver halide photographic emulsion as claimed in claim 1 wherein the emulsion further contains 2-mercaptobenzimidazole, 1-phenyl-5-mercaptotetrazole or 5-methyl-7-hydroxy-1,3,4-triazoindolizine.
 9. A silver halide photographic emulsion as claimed in claim 1 wherein the sensitizing dye is chosen from the group consisting of
 10. A silver halide photographic emulsion as claimed in claim 3, wherein said hydroxyalkyl group is chosen from the group consisting of 2-hydroxyethyl and 4-hydroxybutyl.
 11. A silver halide photographic emulsion as claimed in claim 3, wherein said acetoxyalkyl group is chosen from the group consisting of 2-acetoxyethyl and 3-acetoxypropyl.
 12. A silver halide photographic emulsion as claimed in claim 3, wherein said alkoxyalkyl group is 2-methoxyethyl.
 13. A silver halide photographic emulsion as claimed in claim 3, wherein said carboxy group containing alkyl group is selected from the group consisting of carboxymethyl, 2-carboxyethyl, 3-carboxypropyl, and 2-(2-carboxyethoxy) ethyl.
 14. A silver halide photographic cmulsion as claimed in claim 3, wherein said sulfo-group containing alkyl group is selected from the group consisting of 2-sulfoethyl, 3-sulfopropyl, 4-sulfobutyl, 2-hydroxy-1-sulfopropyl, 2-(3-sulfopropoxy) ethyl, 2-acetoxy-1-sulfopropyl, 3-methoxy-2-(3-sulfopropoxy) propyl, 2-(2-(3-sulfopropoxy) ethoxy) ethyl, and 2-hydroxy-3-(3''-sulfopropoxy) propyl.
 15. A silver halide photographic emulsion as claimed in claim 3, wherein said aralkyl group is selected from the group consisting of benzyl, phenethyl, p-sulfobenzyl, and p-carboxybenzyl. 